Moved yard gem under common group development and documentation in Gemfile

- While running 'bundle' with bundler version 1.5.1 command following message was observed

  Your Gemfile lists the gem yard (>= 0) more than once.
  You should probably keep only one of them.
  While it's not a problem now, it could cause errors if you change the version of just one of them later.

- This commit adds 'yard' gem to common group of development and
  documentation to remove that message